1. Field of the Invention
The present invention relates generally to fiber/resin composites and to methods of making such composites. In a specific aspect, the present invention relates to resin articles comprising arrays of continuous filaments, such as are formed by filament winding, prepregs and the like.
2. Description of the Related Art
In the field of composite materials, a variety of fabrication methods and techniques have come into usage for producing fiber-reinforced resin matrix materials. Continuous filament processes have evolved which are adapted to automated production of filament-reinforced resin articles. The continuous fiber processes include filament winding, wherein the filament in the form of discrete strands or roving is coated with a resin, then wound on a mandrel at a predetermined angle and winding thickness to yield composite articles having high strength when the resin borne on the filament is cured.
In order to have commercial utility the polymeric resins employed in filament winding operations must exhibit low initial viscosity and long pot-life in the process systems in which they are employed. Low viscosity is required in order that deposition of the resin on the filament be highly uniform in character, as is required to achieve substantially uniform properties in the final product article. If viscosity changes appreciably during the filament winding operation, the applied resin thickness may change significantly, resulting in localized stresses or discontinuities in the final product article, product articles which are not within required dimensional tolerance specifications, and inadequate curing of the resin. In addition, the tensional forces on the resin impregnated filaments being processed will significantly increase as the resin viscosity increases, to such extent that the filament becomes highly susceptible to snapping, i.e., tensionally breaking.
Long pot-life of the resin is particularly necessary infilament winding operations where processing times may be on the order of hours. Since the resin is continuously being applied to the filament in these processes, the resin bath or other source of the resin must be continually replenished with resin coating material, and it is therefore necessary that the resin not "set up" or gel in the source bath or other source container and applicating means.
For example, in the fabrication of rocket motors, a resin-bearing filament is wound onto a solid rocket fuel body. In such applications, since the filament winding operation may take upwards of 6 hours and since viscosity must be substantially stable during this period, a long pot-life resin is essential, and consequently the filament wound body must be rotated until full cure of the resin is achieved, which in the case of conventional epoxy resins can range from hours (for heat cured resins) to days (for resins cured at ambient temperatures). Continuous rotation of the mandrel and filament winding is essential in such cases, since cessation of rotation would result in the viscous resin sagging and dripping under gravitational forces, resulting in a resin-rich lower portion of the product article and a resin-poor upper portion of the product. Accordingly, it is desirable to cure the fiber array quickly once it has been formed.
The difficulties inherent in balancing the properties of long pot-life and a quick and easily controlled cure have resulted in the development of numerous types of resins. And within each class of resin, attempts have been made to vary the conditions under which the resins will cure effectively. The standard resins which have been employed in continuous filament processes, as well as in other systems of fiber/resin composite manufacture, generally have deficiencies which have specifically limited their utility in these processes.
The epoxy resins form an extremely important and versatile class of resins. These resins exhibit excellent resistance to chemicals, will adhere to glass and a variety of other materials, show electrical insulation properties, and are relatively easy to use. Among the epoxy resins, systems employing epoxy compounds in conjunction with olefinically unsaturated compounds have found wide acceptance in the art. In particular, resins comprising epoxies and acrylates have been found to be especially useful. This class of resins includes blends of epoxies and acrylates ("epoxy/acrylate" resins) as well as compositions wherein the principal resin component is an acrylic acid-modified epoxy wherein some or all of the epoxy groups have been consumed to produce unsaturated resins. Partially acrylated epoxies are occasionally identified as "dual-functional" compounds since they are designed to exhibit both epoxy and acrylate functional groups on the same molecule.
Within the aforementioned class of epoxy/acrylate systems, compositions have been generated which are adapted to various cure conditions. Such compositions have employed heat curing mechanisms, actinic radiation curing mechanisms, or a combination of both.
Heat curing alone has several disadvantages including reducing the viscosity of the resin, causing it to become more fluid and thereby making it more difficult to handle the article, as well as more difficult to achieve a product of isotropic character. In applications such as filament winding, this drop in viscosity results in resin drip, as previously mentioned. Yet heat curing of epoxy/acrylate systems is an effective and practical means of curing the resins to the fully hardened state that is the source of the resins' great utility.
Heat cured epoxy resin systems are disclosed by U.S. Pat. Nos. 3,408,422 to May et al., 3,441,543 to Heilman, RE 27,973 (3,594,247) to Pennington et al., 3,678,131 to Klapprott et al., 4,017,453 to Heilman et al., 4,025,578 to Siebert, 4,447,586 to Shimp, 4,515,737 to Karino et al., and 5,011,721 to Decker et al.
Exemplary of heat cured compositions are the compositions of U.S. Pat. No. 3,408,422 to May et al. May et al. discloses compositions of an acrylated epoxy polymer and a hydroxylamine (as a stabilizer), as well as unsaturated monomers and peroxides having decomposition temperatures below 150.degree. C. The compositions described by May et al. are heat cured, and include curing agents such as "onium" salts.
The use of actinic radiation to cure or partially cure, i.e., gel the resin, can substantially increase the viscosity of the resin on the formed article. Actinic radiation generally cannot induce complete hardening of the resin and such systems usually employ a catalyst and/or a heat cure step to fully cure the resins. An example of such a process is U.S. Pat. No. 4,892,764 to Drain et al. which employs ultraviolet (UV) light induced polymerization, and requires additional curing at ambient temperatures for extended periods. The Drain et al. patent also employs an aliphatic diamine catalyst which significantly reduces the pot-life of the uncured resin. While the compositions of the Drain et al. patent exhibit some of the desirable resistance to the drip and sag of resin under the forces of gravity, this is due to the fact that they are designed to be cured at room temperature. The Drain compositions are not intended to be heat-cured and as a result exhibit low glass transition temperatures (T.sub.g), thereby having limited utility in applications where the temperature resistance of the cured resins is critical.
Other UV curing systems are found in U.S. Pat. No. 3,922,426 to Feltzin describing filament wound articles impregnated with an ultraviolet light curable resin comprising an unsaturated polyester, an unsaturated monomer, an organic peroxide, and a photosensitizer. More specifically, Feltzin discloses organic peroxides with half-lives at temperatures between 26.degree. C. and 172.degree. C. Other filament winding systems using UV or other actinic radiation to cure resins include U.S. Pat. Nos. 3,660,144, 3,660,145 and 3,660,371 to Johnson et al., 3,772,062 to Shur et al., and 4,479,984 to Levy et al.
Traditionally, dual-curing epoxy/acrylate systems, i.e., systems which employ both an initial actinic radiation exposure and a subsequent thermal polymerization step, have been used for numerous purposes including adhesives, coatings, and prepregs such as those involving filament winding. Such dual-curing prepreg compositions have employed blends of epoxies and acrylates, epoxy curing agents and photoinitiators.
Dual-curing compositions of this kind are described in U.S. Pat. No. 4,092,443 to Green. Green discloses dual-cured filament impregnating resin compositions including a heat curable epoxide or epoxide-containing compound, a photopolymerizable component, such as acrylates, methacrylates and other polyolefinically unsaturated compounds. Heat activated curing agents such as amines, boron trihalides, imidazoles, and anhydrides, as well as optional use of photocatalysts, such as phenones and photo-activated organic peroxides, are also disclosed. The Green compositions, however suffer from a number of the disadvantages associated with dual-cured systems. Principally, these compositions provide little or no resistance to resin drip during the heating step. Articles formed from the Green compositions and process therefore require rotation during heating in order to retain uniformity of resin distribution and the isotropic characteristics and properties dependent thereon.
U.S. Pat. No. 3,937,855 to Gruenwald describes the impregnation of insulated electromagnetic coils with dual-curing resin compositions. The preferred compositions are polyesters solubilized in unsaturated monomers and mixed with peroxides activated by high temperatures along with accelerators such as a tertiary amine or an organo-cobalt compound. The resins described by the Gruenwald patent are quickly gelled by exposing the surface of the applied resin to a highly reactive chemical cross-linking agent, i.e., organic peroxides such as methyl ethyl ketone peroxide, cyclohexanone peroxide, diacetyl peroxide, dilauryl peroxide, cumyl hydroperoxide and benzoyl peroxide. Alternatively, the quick gelation of the resin at the periphery at ambient temperature can be accomplished by incorporation of a photo activator such as a benzoin ether and exposure of the resin to UV light.
U.S. Pat. No. 4,230,766 to Gaussens et al. discloses dual-curing compositions of (meth)acrylated epoxy resins, unsaturated monomers, photoinitiators, and organic peroxides. The resins of the Gaussens et al. patent are cured first by ultraviolet light exposure and heat exposure as a second cure step. The peroxides disclosed by Gaussens et al. include lauroyl peroxide and benzoyl peroxide.
U.S. Pat. No. 3,935,330 to Smith et al. describes dual-curing resins including polyepoxide monomers or polymers, urea/formaldehyde resins, or a melamine/formaldehyde resin, and a thermally curable cross-linker. The compositions described by Smith et al. may also include a dual-functional (meth)acrylamide having at least one double bond and at least one oxirane group. Another component of the compositions includes an ultraviolet light sensitive acrylate. Free radical initiators are included such as organic peroxides including di-t-butyl peroxide, benzoyl peroxide, t-butyl hydroperoxide, perbenzoic acid, and t-butyl peracetic acid. Smith et al. also disclose photosensitizers including phenones. The compositions disclosed by Smith et al. are described as being cured by an ultraviolet light exposure and are subsequently exposed to heat.
None of the aforementioned patents disclose dual-curing filament winding or prepreg resin compositions resistant to the resin sag and drip caused by heat curing. Other measures have generally been needed including spraying a curing agent onto an uncured wound article, e.g., U.S. Pat. No. 3,937,855 to Gruenwald or, more commonly, requiring that the wound article be rotated during the heat cure.
Therefore, it would be a significant advance in the art to overcome the above-described difficulties associated with filament winding processes, in a manner which would obviate the use of additional curing steps and long rotation periods heretofore necessary to obtain quality composites having uniform characteristics.
The present invention solves the disadvantages inherent in the prior art by providing compositions that maintain stable low pot-life viscosities for a significant period of time such that commercial filament winding processes are practicable. The compositions of the present invention also exhibit relatively high glass transition temperatures and are intended to be useful in high temperature applications. Unexpectedly, the resin compositions of the present invention allow uniform properties of the cured product to be obtained without drip or excessive flow of the resin during the heat-cure stage.
Accordingly, it is an object of the present invention to provide an improved process for forming fiber/resin composites.
It is a further object of the invention to provide an improved process for filament winding which overcomes the above-described deficiencies of the prior art practice of these processes.
It is another object of the invention to provide filament wound articles which are readily and economically formed, and which are rapidly processed for subsequent handling, packaging, or other processing operations.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.